Many fabrics, both woven, looped and non-woven, have a surface which presents free ends of fibres generally normal to the plane of the fabric. Such fabrics include felted materials where fibres in a randomly orientated mass are compressed, optionally in the presence of a bonding agent such as an adhesive; materials woven from strands made up from a plurality of individual fibres where the surface of the fabric has been, brushed, teased, abraded or otherwise treated to separate some of the fibres from within the strands to form a fluffy surface to the material, for example a brushed nylon; woven materials made from materials which are inherently fluffy, such as knitted or woven angora, merino or cashmere wools or cotton terry towelling; and carpet type materials such as velvets, velours and tufted carpets where individual lengths of strands or fibres are knotted, sewn, glued or otherwise secured to a sheet member, typically a reticulate backing sheet, whereby the free ends of the strands or fibres form a pile which extends generally normal to the plane of the backing or where loops of the strands or fibres are formed extending generally normal to the plane of the backing and the free ends of the loops may be severed to form the pile. For convenience the term pile fabric will be used herein to denote all such types of material where individual fibres or strands comprising groups of fibres extend generally normal to the plane of the material to provide a pile effect surface to the material.
It is often desired to form patterns or images upon the surfaces of pile fabrics, for example a coloured pattern. This can be achieved by interweaving different coloured, textured or other material strands of wool or other material into the fabric as it is being made. However, this is difficult and time consuming, especially where the pattern is complex and/or a plurality of colours or textures are desired. Such use of a plurality of different strands is becoming progressively uneconomic in the large scale manufacture of commodity materials, such as patterned carpets.
It has therefore been proposed to manufacture the pile fabric from neutral or uniformly coloured fibres or strands and to apply a colour to the pile fibres after the fabric has been woven or otherwise manufactured. The colour is typically an ink applied by any suitable printing technique. A printing technique which is used is an ink jet printing technique using a drop on demand type of printer in which ink is ejected through a collection of nozzles each attached to a valve mechanism serving each nozzle. The opening and shutting of the valves is under the control of a suitable computer so that the valves are operated for the required duration and in the required sequence to produce the desired printed pattern on the fabric. However, problems arise in securing even application of the printing ink to the individual strands or fibres of the pile. The ink is desirably applied to carpets at the rate of about 300 to 400% by weight of the fibre to be coloured and needs to penetrate substantially uniformly throughout the strands formed from the individual fibres. If a very mobile ink having a viscosity of about 10 cPs at 25° C. (as is commonly used in an ink jet printer) is used, it will run down the length of the strands and form an intense coloration at the base of the pile, leaving the top portion of the pile inadequately dyed, and little penetration of the colour into the strands will take place. It is therefore necessary to increase the viscosity of the ink in order to ensure that it runs down the fibre at a sufficiently slow rate for uniform penetration of the ink into the strands and coverage of the surface of the individual fibres takes place. The longer the pile, the greater this problem becomes and with long pile fabrics, that is those with a pile length of about 2 mms or more, it is necessary to use a very viscous ink having a viscosity of from up to 500 cPs at 25° C.
Such viscous inks are difficult to jet through the very fine orifice nozzles, typically less than 500 micrometer diameter, and pressures far in access of those for which a valve ink jet printer is normally designed would be required. Furthermore, if a low viscosity ink were applied at such high pressures, it may issue from the nozzles as high powered jets and cause the individual strands to bend over and thus prevent the ink from contacting other strands in the pile. It is therefore customary to use nozzles having orifices which are progressively greater in diameter as the length and closeness of the pile increases. Thus, for a loosely packed pile in a Terry towel fabric, it may be possible to achieve satisfactory printing using inks with a viscosity of 6 to 15 cPs and a pressure of 1.5 to 2 bar through a nozzle of 60 micrometers diameter. However, with a heavier upholstery type of fabric which typically has a pile length of typically 3 to 5 mms, it may be necessary to use an ink with a viscosity of about 120 cPs, a pressure of 1.5 to 2.5 bar and nozzle diameters of 90 to 150 micrometers. With a carpet having a pile length of 15 mm or more it is necessary to use an ink having a viscosity of up to 500 micrometers, a pressure of about 2 bar and nozzle diameters of typically 500 micrometers in diameter so that the viscous ink can be ejected in sufficient amounts to attain the desired loading of ink on the individual strands.
Whilst the use of large diameter nozzles for high viscosity inks enables the ink to be deposited on the strands of the pile to achieve substantially uniform coloration of the individual strands and fibres, the size of the droplets issuing from the nozzle are sufficiently large to cause perceptible loss of definition in the printed pattern. Furthermore, the size of the droplets can result in adjacent droplets applied to the pile contacting one another to cause colour bleeding where the droplets are of different colours.
We have determined that the use of a drop on demand print head which operates at frequencies greater than 1 kHz enables the size of the droplets being printed to be reduced, which reduces the problems of colour bleed and enhances the definition of the printed image or pattern without reducing the print rate below commercially practical levels. Furthermore, we have found that it becomes possible to omit individual printed droplets from the printed pattern and thus print a blank within the image which is not visually perceptible but which acts to provide a gap within the printed strands to act as a barrier to colour bleeding. Such a gap may also be printed as a black line defining the edges of areas printed with different colours, which enhances the perceived definition of the printed image or pattern.
It has also been discovered that drop-on-demand ink jet printers are also suitable for printing applications in which colour needs to be applied to a thin fabric, such as, for example, a polyester mesh, with the result that the colour on each side of the fabric is of equal brilliance. Such an effect is a requirement when printing flags and banners, for example. Most known ink jet printing machines used to print onto light textiles tend to utilize impulse jet printing technology that, whilst producing high definition printing, lays down very small amounts of ink in a dotting fashion. This tends to result in pale colours being printed unless multiple print passes are used which significantly slows down the linear print rate. The other disadvantages associated with impulse jet printing technologies include the low pressures at which impulse jet printers operate and the low viscosity inks that must be used with the printers. This is a particular disadvantage for textile printing as higher viscosity inks are preferred as they provide a more consistent depth of colour pick up through the textile, which is vital to obtaining quality print. Higher viscosity fluids also prevent sideways wicking (“bleeding”) of fluid, thus avoiding poor colour definition and/or unintended colour “blending” at colour change points in the printed pattern.
In order to obtain bright colour quality and achieve a consistent depth of colour through the textile pile at high print speeds, it is preferred to have the option to apply the ink by opening the printing nozzle orifice and keep it open for a period of time sufficient to form a stripe of a desired length. This technique is referred to as dosing. This process is different to producing a multiplicity of dots in a linear print fashion and ensures continuous, consistent colour dosage through the textile fibres.
It has been found that a drop on demand print head, in which individual valves are mounted and fed from a common, chamber via a plug in system can be arranged to produce both dot and dosing print effects. An important feature of this arrangement is that it can use high viscosity inks. A further advantage of the arrangement is that the design of the drop on demand head enables fluid pressures of up to 3.5 bar to be used. Impulse jet heads do not have the capability for higher viscosity inks or high pressure application. A further advantage of individually plug in mounted and supplied valves is the significant reduction in space needed to present requisite number of print orifices, thereby enabling higher definition dot printing. The provision of individual valves which can be fitted in this manner also enables fast and economical valve replacement for maintenance purposes as the plug in facility provided for fast and accurate fluid and electrical connection. The combination of higher viscosity inks (for example 12 cp or greater) and high pressure application enables the provision of a printing capability that can not be achieved using other known technology.
A further advantageous feature of the drop on demand arrangement is the ability to pass higher viscosity fluids through the arrangement without incurring flow problems and pressure drops across a bank of nozzle orifices. This is achieved through a combination of methods of fluid feed and internal channel design. This feature mitigates the effect referred to as ‘banding’, in which a striping effect can be seen on the printed textile surface if pressure drops occur at the nozzle orifices, particularly when the print head is dosing. A significant additional benefit of operating in a dosing mode is the reduction in mechanical wear of the moving parts of the valve due to less open and close operations than would otherwise be required in a rapid dotting mode. A further advantage is the relative ease in which constant pressure can be maintained across all adjacent orifices.
The drop on demand arrangement can be configured such that multiple modules (for single or multiple colour printing) can be mechanically arranged to provide larger area of print coverage with precise alignment of the nozzle orifices. In this way, for example, a bank of modules can be arranged to provide wide coverage for individual colours, whilst physically aligned with further banks of modules for other colours. An important design feature is that modules can easily be mounted to achieve a “seamless” bank of nozzle orifices with accurate pitching of adjacent modules relative to each other. Furthermore, these banks of modules can also be manifold fed with ink such that one bank of modules can be dedicated to one colour. Each bank of modules is attached to a mechanical mounting system that can be arranged so that individual modules can be replaced without disturbing the ink supply tubing or drive electronics, enabling the simple maintenance and/or replacement of modules.
It should also be noted that when printing in a dosing mode it is still necessary to provide very accurate control of the valve such that the line produced by the dosing sequence has a sharply defined cut off at its start and end points. Such fine control is achieved with this invention.
The design of the printer module is such that the main nozzle chamber can be heated or cooled with accurate temperature control. The benefit of this feature is that the jetting viscosity of some textile ink types can be modified at the point of jetting to achieve best jetting performance whilst retaining best colour penetration performance on the textile substrate. Typically this would be the case where a high viscosity ink is required for colour penetration needs and the jetting of the ink is best performed through reducing the viscosity through heat application. Alternatively, a cooling mode could be used where excess heat in the system can be controlled to ensure the jetting viscosity is not too low to ensure best colour penetration on the substrate.